In a free electron laser (FEL) an electron beam is directed through a wiggler which includes a series of magnets with alternating poles that bend the electron beam in a sinusoidal path to generate laser light. A cookie-cutter 26, such as shown in FIG. 1, is positioned at the upstream end of the wiggler to cut the electromagnetic wake field wave advancing through the beam tube's round aperture into the slot shaped wave entering the slot aperture of the wiggler.
Referring to FIG. 1, the cookie-cutter 26 includes a body or thermal mass extension 28 and a nose portion 30. The nose portion 30 includes a wide slot 32 that extends longitudinally through the body 28 and matches the wiggler chamber's aperture. The wide slot 32 conforms substantially to the 3:1 rule for electron beam impedance control through the wiggler. The shape of the slot and the width (W1, see FIG. 6) being substantially 3 times the height (H1, see FIG. 6) of the slot 32 is critical to minimizing beam impedance through the wiggler. The cookie-cutter is preferably machined from a single piece of oxygen-free high conductivity (OFHC) copper. OFHC copper includes a substantially high thermal conductivity to drain away deposited heat. This copper also has a high level of chemical purity beneficial in high vacuum applications. A plurality of vent slots 34 extend into the nose portion 30 and the cookie-cutter body 28 in order to provide vacuum conductivity. The cookie-cutter 26 includes an upstream end 36, a downstream end 38, and a substantially cylindrical body 28. The nose portion 30 includes a flat top 40 surface and flat bottom 42 surface. The flat top and bottom surfaces 40 and 42 terminate in a planar face 44 on the body 28 that is substantially perpendicular to the longitudinal axis 46 of the cookie-cutter 26. A plurality of threaded mount holes 48 are provided, extending into the body 28 from each planar face 44 of the cookie-cutter.
Although the cookie-cutter controls the wake field wave entering the wiggler's slotted aperture, the remaining cut-off portions of the wake field are still active and will bounce back as broad band microwave and TeraHertz (THz) radiation toward the upstream and, if not damped, will interfere with the forward moving wake field of the oncoming electron beam pulse. The wake field interference broadens the electron beam pulse's narrow longitudinal energy distribution and diminishes ultimate laser power. Thus there is a need to minimize the amount of bounced back radiation returning from the outer edges of the cookie-cutter that defines the wiggler chamber aperture.